The present invention relates to electrically conductive cements or adhesives and, more particularly, to electrically conductive cements or adhesives having superior long-term performance in high temperature and high humidity environments.
Electrically conductive cements and adhesives are typically fabricated from single- or multi-component non-conductive carrier materials and particulate metallic fillers. While various cements can be used as the carrier, multi-component epoxies and single-component solvent-based systems are preferred. Epoxies have a long shelf life, can be cured with relative ease, and form a strong and reliable bond with many materials. In a similar manner, single-component solvent-based systems can be readily cured by driving off the solvent to form a strong and reliable bond with many materials. The metallic fillers are typically noble metals, such as gold or silver, in various particulate sizes. The preferred metallic filler is typically a mixture of flake-like and non-flake particles of various sizes. The particles can be essentially solid or, in some cases, metal-plated non-conductive bodies. In a typical formulation, the conductive filler can comprise 75% by weight of the total material with the polymeric carrier comprising the remaining material. It has been thought that metal particles in the form of flakes or platelets provide the bulk conductivity characteristics because the flakes tend to align themselves in an overlapping relationship in the cured carrier to provide a conductive electron pathway. Non-flake conductive fillers in the carrier are thought to fill interstices between the flake-like particles in the carrier to provide enhanced conductivity between the flake- or platelet-like particles.
Most electric circuits are fabricated as traditional rigid printed circuit boards using the subtractive process in which copper traces and connection pads are etched from a copper foil layer attached to a rigid, non-conductive board. The solder-plated leads of the electrical components are passed through mounting holes in the board and connected to their connection pads by lead/tin soldering. It is also known to fabricate so-called flex circuits in which the copper traces and connection pads are formed on a flexible layer of a polyester material in the 2-3 mil thickness range, such as KAPTON.TM. mylar. The leads of the electronic components are passed through mounting holes in the flexible substrate and connected to their connection pads by lead/tin soldering in a manner analogous to the rigid circuit boards. Recently, surface-mount components (SMC) have been developed in which the component leads are merely mounted upon the conductive pads and soldered into place to form a butt joint; surface-mount technology being suitable for both rigid and flexible substrates. Traditional soldered-component systems represent proven and highly developed technologies with proven performance under various temperature/humidity conditions. However, traditional soldered-component systems require extensive chemical processing with various types of etchants and similar chemicals to fabricate the circuit substrate and various fluxes and solvents to effect the soldered connection. Additionally, soldering involves the application of heat to effect the soldered connection. While components and rigid substrates are designed to accommodate the heat of soldering, flexible circuits are more susceptible to distortion because of their relatively thin cross-section and low heat capacity. Thus, soldering components on a flexible substrate can cause local 'puckering' of the substrate, changes in the center-to-center dimensions of the various connection pads, and warpage of the entire circuit substrate.
Efforts have been made to use electrically conductive inks, cements, and adhesives to replace existing soldered-component systems in both rigid and flexible substrate applications in an effort to reduce costs and reduce the adverse effects of the chemicals and heat used in soldered-component systems. For example, electrically conductive inks have been printed on flexible polyester substrates and the conductive leads of the surface-mount components then cemented to their connection pads by electrically conductive single-component adhesives or multi-component epoxy resins. The resulting printed circuit can be readily configured to fit into a particular mounting envelope in a manner that cannot be achieved with rigid printed circuit boards. It is estimated that a successful system utilizing conductive inks and epoxies will provide significant cost savings relative to traditional soldered-component systems.
While electrically conductive cements do not possess the conductivity of solid metals and solder alloys, their conductivity (for example, 0.1-2 .OMEGA. per connection) is adequate for many electrical circuits. For example, a junction resistance of one ohm or so will have little effect where the circuit component is a resistor or other circuit device having a resistance or impedance of several hundred or thousands of ohms or greater. While junction resistance becomes more important in low impedance circuit applications, the circuit can oftentimes be designed to accommodate cumulative junction resistances.
One factor that affects the electrical conductivity of the conductive cement that defines the junction is the presence or absence of non-conductive or resistive surface oxides that form as a consequence of exposure to ambient air and moisture. In soldered-component systems, this oxide is largely removed by the heat of the molten solder and the use of various types of fluxes that shield the junction from the ambient atmosphere and moisture as the solder cools. In conductive cement systems, in which heat and fluxes are not present during the curing process, it is preferable that any cement or adhesive have a mechanism that can overcome surface oxide without the need to clean the leaded electrical components with aggressive cleaning agents prior to effecting the connection.
While existing conductive cements and adhesives provide adequate performance for most applications, all such cements and adhesives are susceptible to changes in their resistivity .rho. with continued exposure to humidity and are particularly sensitive to continued exposure to the combination of higher humidities and higher temperatures. Since the cements use a single- or double-component polymeric binder as the curable adhesive medium, they are all permeable, to some extent or the other, to moisture. While a circuit can be designed to accommodate cumulative junction resistances, any change in that resistance with time will probably have a detrimental affect on overall electrical performance. It is believed that moisture permeates the polymeric binders with time to effect oxidation at the connection interface between the connection pad and the component lead, which oxidation tends to increase resistance and diminish the probability of maintaining a gas-tight seal. Where silver is used as the conductive filler, the presence of moisture also accelerates the development of silver ions, and, under the certain circumstances, can foster undesired migration.
In general, silver-filled polymeric systems perform well over a reasonably large temperature range but not at high humidity. When aged under high humidity conditions, the resistance of the junction oftentimes increases significantly. While many circuits can operate adequately with increases in the resistively of one or more of their connections, the humidity sensitivity is considered a factor limiting more widespread use of conductive polymeric cements and adhesives in rigid and flexible substrate applications.
As a representative example of the change in junction resistance under 90% relative humidity conditions, a conductive adhesive manufactured by Emerson & Cuming of Lexington, MA 02173 and sold under the AMICON.TM. CSM-933-65-1 designation was used to connect a 68-pin surface-mount device (SMD), two 44-pin surface-mount devices, and a ten-resistor series string in a test circuit and subjected to a 140.degree. C. cure for a period of 10 minutes in accordance with the manufacturer's instructions. The pins of the various surface-mount devices were series-connected through resistive elements within the surface-mount devices and the total junction resistance determined by subtracting the cumulative series resistance of the intra-device elements from the total measured resistance. In a similar manner, the resistance measurement for the resistor-string was effected by subtracting the cumulative values of the resistors that comprised the string from the total series resistance to arrive at a junction resistance value. The resistance of the intra-device elements of the surface-mount devices as well as the resistors within the resistor string was stable within the temperature and humidity range of the tests as verified by control circuits. The initial junction resistance in ohms was measured at room temperature and at test conditions of 60.degree. C. and 90% relative humidity as shown in the left two column of FIG. 1. The resistivity was then again measured after 15.5, 24, 39, 63, and 64 hours exposure at the 60.degree. C. and 90% relative humidity test conditions. As shown in FIG. 1, the ohmic resistance of all junctions increased with time at 90% relative humidity with an extrapolated value for 1000- hours indicating a substantial increase for the 68-pin devices and the two 44-pin devices and less of an increase for the resistor-string.
As can be appreciated from a consideration of the data in FIG. 1, a known conducive adhesive undergoes a substantial increment in junction resistance at elevated relative humidity with time, this increase sufficient to mitigate against the use of the material in many circuit applications.